I Am The Center Of Gravity, Hold A Capital Situation In Vienna, And As I Am Foremost In Every Victory

Riddle: I am the center of gravity, hold a capital situation in Vienna, and as I am foremost in every victory, am allowed by all to be invaluable. Though I am invisible, I am clearly seen in the midst of a river. I could name three who are in love with me and have three associates in vice. It is vain that you seek me for I have long been in heaven yet even now lie embalmed in the grave. What am I?